The company, in a statement, confirmed that hackers broke into their system and stole an unknown quantity of data, reported Entertainment Weekly.
"HBO recently experienced a cyber incident, which resulted in the compromise of proprietary information. We immediately began investigating the incident and are working with law enforcement and outside cybersecurity firms.
"Data protection is a top priority at HBO, and we take seriously our responsibility to protect the data we hold," the network confirmed in a statement.
So far, written material, which is believed to have been from "Game of Thrones" season seven's fourth episode and an upcoming episode of "Ballers" and "Room 104" have apparently been put online.
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"As most of you have probably heard by now, there has been a cyber incident directed at the company which has resulted in some stolen proprietary information, including some of our programming.
HBO had also faced a leak during "Game of Thrones" season five premier, when the first four episodes of the show landed online. But this is the first major cyber attack on the company.
On Sunday, an anonymous email was sent to reporters announcing the hack: "Hi to all mankind. The greatest leak of cyber space era is happening. What's its name? Oh I forget to tell. It's HBO and 'Game of Thrones'...!!!!!! You are lucky to be the first pioneers to witness and download the leak. Enjoy it and spread the words. Whoever spreads well, we will have an interview with him. HBO is falling."
In April, episodes from season five of "Orange Is The New Black" were put online, ahead of its premiere on Netflix.
In May, a hacker claimed to have stolen "Pirates of the Caribbean: Dead Men Tell No Tales" from Disney ahead of the film's release though it later turned out to be a hoax.
Sony has so far faced the biggest Hollywood hack when in 2014, hackers stole 100 Terabytes of data and uploaded it online.
Sony Pictures Entertainment Co-Chairman Amy Pascal resigned after her controversial emails leaked online.
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